1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for efficiently using resources for control information transmission.
2. Description of the Related Art
In communication systems, for example, LTE (Long Term Evolution) system, Acknowledgement (ACK)/Negative ACK (NACK) feedback information, a type of uplink control information, which is a signal used for indicating success/failure in receiving downlink transmission data to which Hybrid Automatic Repeat reQuest (HARQ) is applied.
In the LTE system, a transmission format for uplink control information is classified according to the presence/absence of uplink transmission data. When simultaneously transmitting data and control information over the uplink, the LTE system Time Division Multiplexing (TDM)-multiplexes the data and control information, and maps the data and control information to time-frequency resources allocated for data transmission before transmission. However, when transmitting only the control information without data transmission, the LTE system uses an allocated particular frequency band(s) for control information transmission. Currently, in the standard conferences, Physical Uplink Control Channel (PUCCH) is being defined as a physical channel for transmitting only control information, and the PUCCH is map to the allocated particular frequency band.
FIG. 1 is a diagram illustrating a transmission structure of a physical channel PUCCH for control information transmission over the uplink in a 3GPP LTE system. In FIG. 1, the horizontal axis represents a time domain, and the vertical axis represents a frequency domain. More specifically, FIG. 1 illustrates the time domain in one subframe 102, and the frequency domain in a system transmission bandwidth 110.
Referring to FIG. 1, the subframe 102, which is a basic transmission unit of the uplink, has a length of 1 ms, and one subframe includes two slots 104 and 106, each having a 0.5-ms length. Each slot 104 and 106 includes multiple SC-FDMA symbols 111-123 (131-143). FIG. 1 illustrates an example where one slot includes 7 SC-FDMA symbols.
In the time-frequency domain, a basic unit of resources is a Resource Element (RE). The RE can be defined by a SC-FDMA symbol index (OFDM symbol index, for downlink) and a subcarrier index. A basic unit of resource allocation is a Resource Block (RB), such as RBs 108 and 109. The RBs 108 and 109 include multiple subcarriers and multiple SC-FDMA symbols. In the example illustrated in FIG. 1, 12 subcarriers and 14 SC-FDMA symbols constituting 2 slots constitute one RB.
Referring to FIG. 1, a frequency band to which the PUCCH is mapped, corresponds to reference numeral 108 or reference numeral 109 representing an RB corresponding to one of both ends of the system transmission band 110. The PUCCH can apply frequency hopping to increase frequency diversity during one subframe, and in this case, slot-by-slot hopping is possible. A base station (or a Node B) can allocate multiple RBs for transmission of the PUCCH to approve transmission of control information from multiple users.
Referring to FIG. 1, control information #1, which was transmitted over the pre-allocated frequency band 108 in the first slot 104, is transmitted over another pre-allocated frequency band 109 in the second slot 106, after undergoing frequency hopping. However, control information #2, which was transmitted over the pre-allocated frequency band 109 in the first slot 104, is transmitted over another pre-allocated frequency band 108 in the second slot 106 after undergoing frequency hopping.
In the example illustrated in FIG. 1, in subframe 102, control information is transmitted in SC-FDMA symbols represented by reference numerals 111, 112, 116, 117, 138, 139, 143, and 144, or reference numerals 131, 132, 136, 137, 118, 119, 123, and 124, and pilots (or Reference Signals (RSs)) are transmitted in SC-FDMA symbols represented by reference numerals 113, 114, 115, 140, 141, and 142, or reference numerals 133, 134, 135, 120, 121, and 122. The pilot, which includes a predetermined sequence, is used for channel estimation for coherent demodulation at a reception side. The number of SC-FDMA symbols for control information transmission, the number of SC-FDMA symbols for RS transmission, and their positions in the subframe are all given herein by way of example, and these are subject to change according to the type of desired transmission control information and/or system operation.
Normally, Code Division Multiplexing (CDM) is used to multiplex ACK/NACK transmitted over a PUCCH for different users in the same RB. As described above, the base station can allocate multiple RBs for transmission of the PUCCH to approve transmission of the ACK/NACK from multiple users.
A Zadoff-Chu (ZC) sequence is under discussion as a sequence to be used for CDM of the control information. The Zadoff-Chu sequence, as it has a constant signal level (or a constant envelop) in the time and frequency domains, has a good Peak-to-Average Power Ratio (PAPR) characteristic and shows excellent channel estimation performance in the frequency domain.
Generally, a length of the Zadoff-Chu sequence used for the PUCCH is assumed to be 12 samples, the number of which is equal to the number of subcarriers included in an RB. In this case, because the maximum possible number of different cyclic shift values of the Zadoff-Chu sequence is 12, it is possible to multiplex a maximum of 12 PUCCHs to one RB by allocating different cyclic shift values to the PUCCHs.
FIG. 2 illustrates an example of multiplexing an ACK/NACK of each user in the same RB with different cyclic shift values of the Zadoff-Chu sequence in transmitting the ACK/NACK over a PUCCH having the above-described structure. In FIG. 2, the vertical axis represents cyclic shift values 200 of a Zadoff-Chu sequence. In the Typical Urban (TU) model considered as a wireless channel model, because the maximum possible number of channels that can undergo multiplexing in one RB without abrupt loss of orthogonality is 6, FIG. 2 illustrates a case where 6 ACK/NACK signals 202, 204, 206, 208, 210, and 212 undergo multiplexing. FIG. 2 illustrates an example of using the same RB and the same Zadoff-Chu sequence for transmission of the ACK/NACK in such a manner that ACK/NACK 202 from UE #1 is transmitted using a cyclic shift ‘0’ 214; ACK/NACK 204 from UE #2 is transmitted using a cyclic shift ‘2’ 218; ACK/NACK 206 from UE #3 is transmitted using a cyclic shift ‘4’ 222; ACK/NACK 208 from UE #4 is transmitted using a cyclic shift ‘6’ 226; ACK/NACK 210 from UE #5 is transmitted using a cyclic shift ‘8’ 230; and ACK/NACK 212 from UE #6 is transmitted using a cyclic shift ‘10’ 234.
It is possible to increase multiplexing capacity of the PUCCHs carrying control information by additionally applying time-domain orthogonal sequences in addition to the CDM control information transmission based on the Zadoff-Chu sequence.
A typical example of the orthogonal sequence includes a Walsh sequence. For length-M orthogonal sequences, there are M sequences satisfying orthogonality therebetween. More specifically, for 1-bit control information, such as ACK/NACK, its multiplexing capacity can be increased by applying time-domain orthogonal sequences to SC-FDMA symbols to which ACK/NACK is mapped before transmission. An orthogonality loss caused by a change in wireless channels can be minimized by restricting a time interval in which the time-domain orthogonal sequences are applied, to one slot or less. For example, length-4 orthogonal sequences are applied for 4 SC-FDMA symbols for ACK/NACK transmission in the one slot, and length-3 orthogonal sequences are applied for 3 SC-FDMA symbols for RS transmission in the one slot. Basically, for the ACK/NACK and RS, user identification is possible with the cyclic shift values of the Zadoff-Chu sequence, and additional user identification is available by the orthogonal sequences. For coherent reception of ACK/NACK, because an RS(s) is required, which is mapped to ACK/NACK on a one-to-one basis, multiplexing capacity of the ACK/NACK signals is restricted by the RS mapped to the ACK/NACK.
FIG. 3 illustrates an example of multiplexing an ACK/NACK of each user in a same RB with time-domain orthogonal sequences in addition to the different cyclic shift values of the Zadoff-Chu sequence in the PUCCH structure for ACK/NACK transmission. In FIG. 3, the vertical axis represents cyclic shift values 300 of a Zadoff-Chu sequence, and the horizontal axis represents time-domain orthogonal sequences 302. In the TU model considered as a wireless channel model, the maximum number of cyclic shift values with which their multiplexing is possible in one RB, without abrupt loss of orthogonality, is 6, and if 3 length-4 orthogonal sequences 364, 366, and 368 are additionally used, a maximum of 6*3=18 ACK/NACK signals 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324, 326, 328, 330, 332, 334, 336, and 338 can be multiplexed.
FIG. 3 illustrates an example of using a same RB and a same Zadoff-Chu sequence for ACK/NACK transmission in such a manner that ACK/NACK 304 from UE #1 is transmitted using a cyclic shift ‘0’ 340 and an orthogonal sequence ‘0’ 364; ACK/NACK 306 from UE #2 is transmitted using a cyclic shift ‘0’ 340 and an orthogonal sequence ‘1’ 366; ACK/NACK 308 from UE #3 is transmitted using a cyclic shift ‘0’ 340 and an orthogonal sequence ‘2’ 368; through; ACK/NACK 334 from UE #16 is transmitted using a cyclic shift ‘10’ 360 and an orthogonal sequence ‘0’ 364; ACK/NACK 336 from UE #17 is transmitted using a cyclic shift ‘10’ 360 and an orthogonal sequence ‘41’ 366; and ACK/NACK 338 from UE #18 is transmitted using a cyclic shift ‘10’ 360 and an orthogonal sequence ‘2’ 368. The orthogonal sequences 364, 366, and 368, which are length-4 orthogonal codes, satisfy orthogonality therebetween.
The resource information needed in transmitting an ACK/NACK over a PUCCH includes (i) RB information indicating through which RB the ACK/NACK is transmitted, (ii) cyclic shift information of the Zadoff-Chu sequence, and (iii) orthogonal sequence information.
The present invention provides a method for efficiently using resources and improving reception performance by defining the detailed mapping relation between control channels transmitted by the base station and resources for ACK/NACK transmission in enabling the base station and the UE to recognize, in common, the resource information for the UE ACK/NACK transmission.
A basic unit constituting a downlink control channel is a Control Channel Element (CCE). One downlink control channel includes one or multiple CCEs, each of which includes multiple REs. An increase in the number of CCEs of the downlink control channel can reduce a channel coding rate applied to control information mapped to the downlink control channel, making it possible to obtain channel coding gain.
FIG. 4 illustrates an example where a downlink control channel is generated. More specifically, FIG. 4 illustrates an example in which for a total of NCCE CCE 401-409, a downlink control channel 430 for UE #1 is generated with 4 CCEs of CCE#0 401-CCE#3 404; a downlink control channel 422 for UE #2 is generated with 2 CCEs of CCE#4 405 and CCE#5 406; a downlink control channel 416 for UE #3 is generated with 1 CCE of CCE#6 407; a downlink control channel 417 for UE #4 is generated with 1 CCE of CCE#7 408; and a downlink control channel 418 for UE #5 is generated with 1 CCE of CCE#NCCE−1 409. That is, the base station transmits the downlink control information for UE #1-UE #5 in an arbitrary subframe. When multiple CCEs are included in one downlink control channel, the CCEs can be consecutive as illustrated in FIG. 4, or can be scattered, i.e., non-consecutive. The positioning of the CCEs depends on the system operation.
The UE performs decoding on the downlink control channels that can be generated with the total of NCCE CCEs, and upon detecting its own Identifier (ID), identifies that the corresponding channel is a downlink control channel transmitted to the UE itself.